Electric contacts, regularly used in switches and other electromechanical devices, often have nonconductive contaminants on their surfaces which can cause voltage drops of up to 300 millivolts or higher. While such voltage drops are numerically small, they are potentially high enough to impede the operation of the electrical devices being supplied current. As a result, a maximum voltage drop across the electric contacts is a common design requirement for switches and electromechanical devices, particularly those used in automotive applications. Currently, visual inspection is often used to identify the presence of contaminants on electric contacts. However, industry trends towards increasingly stringent voltage drop requirements have necessitated a search for more sophisticated methods of meeting those requirements.
Typical nonconductive contaminants found on electric contacts include oxides which form on the contacts by atmospheric oxidation; traces of chemicals, such as acids, used during milling operations; and protective coatings, such as paraffin wax, which suppliers sometimes use to keep the contacts shiny and to prevent oxidation. Perhaps the most expeditious way to satisfy the voltage drop requirement is to clean the contacts to remove any contaminants before installing the contacts in switches or other devices. Due to the number of variables which can affect the condition of a contact's surface, repeatable, quantitative surface continuity data would aid in the development of contact cleaning processes and the construction of switches and other devices which must meet voltage drop requirements.
Devices for measuring voltage drop or surface continuity, both of which are essentially the same parameter, have long been known. These devices range from simple two-wire "multi-meters," which are readily available at consumer electronics stores, to more sophisticated four-wire devices. Two-wire meters are generally limited to measuring fairly large voltage drops because their probes, which both conduct current through a test sample and measure the voltage drop across the sample, contribute a large resistance to the test circuit, masking small voltage drops in the sample. Four-wire devices, on the other hand, can accurately measure voltage drops on the millivolt level because they use separate probes to supply current to the sample and to measure voltage drop. Generally, two probes supply current, while two different probes measure the voltage drop.
Ideally, voltage drop data should be taken under conditions which directly correlate with those encountered in an end product. Obtaining repeatable, quantitative voltage drop data requires the use of correctly shaped probes and a predetermined contact force between the probes and surface of the sample. This is often difficult to do with existing voltage drop meters which typically lack a means for repeatably applying a desired contact force. Quantitative voltage drop data can be obtained from actual switches, but the use of actual switches to obtain such data is cumbersome because the switches are not easily adapted to testing large numbers of samples.
Accordingly, it would be desirable to have a device which can provide repeatable, quantitative voltage drop data under conditions which directly correlate with those encountered in an end product and which is versatile enough to provide data for large numbers of samples.